Hot surface igniters are used to ignite combustion gases in a variety of domestic and industrial appliances, including furnaces, kitchen ranges, and clothing dryers. The igniters typically include a semi-conductive ceramic body with terminal ends across which a potential difference is applied. Current flowing through the ceramic body causes the body to heat up and increase in temperature, providing a source of ignition for combustion gases.
When in service, igniters are typically subject to variable line voltages. Igniters are typically specified to achieve a certain minimum temperature at a minimum expected voltage (TVmin) within a specified period of time and not to exceed a certain maximum temperature at the maximum line voltage (TVmax). The nominal line voltage that is typically expected to be encountered lies between the minimum and the maximum, and it is often preferred that when operating at the nominal line voltage, the igniter temperature is as close to TVmin as possible, while still exceeding it. For example, in the United States, the nominal line voltage to appliances such as a gas range and a residential furnace is 120V, maximum line voltage is 132V and minimum expected voltage is 102V.
A key property of hot surface igniters is their resistivity, which is an intrinsic property of the igniter material:ρ=R(A/L)  (1)                where, ρ=resistivity (ohm-cm)                    R=resistance (ohms)            L=length (cm)            A=cross-sectional area (cm2)                        
As equation (1) indicates, the length and cross-sectional area can be varied in forming an igniter body out of a given material to obtain a desired resistance. Resistivity is temperature dependent. Thus, a given igniter material will typically exhibit a different resistivity at room temperature and at the service temperature of the igniter (high temperature resistivity).
The ratio of room temperature resistivity to high temperature resistivity is an important igniter property for several reasons. First, if the ratio is too high or low, then room temperature performance will not be a good indicator of high temperature performance. Apart from the ratio, if the room temperature resistivity is too low, the igniter will reach TVmax at a voltage that is less than the maximum line voltage, Vmax. This excessive heating will tend to shorten the igniter life. Apart from the ratio, if the room temperature resistivity is too high, the igniter may not reach the ignition temperature of the gas it is intended to ignite within the desired time frame.
In addition, in the fabrication process, igniters are often slotted to create legs of reduced cross-sectional area. The slotting process is often carried out dynamically by measuring the room temperature resistance as a slot is progressively lengthened until the desired room temperature resistance is achieved. The room temperature resistance is used to shorten the testing time following adjustments to slot length. However, if the room temperature resistance does not correlate well with the high temperature resistance, a given igniter may be slotted incorrectly and unable to achieve the desired temperature performance.
One type of hot surface igniter material that is well known is silicon carbide. The silicon carbide is typically formed into a slurry, shaped into a desired igniter preform shape, and then sintered. The sintering process can be adjusted to achieve desired electrical properties by doping with electron acceptors or donors. In certain known processes, the igniter body is subjected to a reducing atmosphere that comprises nitrogen during the sintering process to adjust the resistivity of the silicon carbide and provide oxidation resistance. During such processes, the silicon carbide vaporizes and recrystallizes with nitrogen incorporated as an n-type dopant into the silicon carbide lattice.
In certain known silicon carbide igniter manufacturing techniques, green (unsintered) igniter bodies are sintered in an inert (nitrogen-deficient) reducing atmosphere in a first relatively higher temperature sintering phase and then sintered in a 100 percent nitrogen sintering atmosphere in a second relatively lower sintering temperature phase. In other known techniques, structural ceramic bodies are sintered in a 100 percent nitrogen sintering atmosphere in a first sintering phase and are then sintered in a partially-nitrogenated, reducing atmosphere in a second sintering phrase. During the second sintering phase, the sintering temperature is ramped to a maximum sintering temperature, at which point the nitrogen content of the reducing atmosphere is reduced until it is entirely inert in a third sintering phase. A fourth sintering phase is then carried out in an inert reducing atmosphere at the maximum sintering temperature. Such known sintering techniques are generally incapable of providing igniters with the desired electrical properties for certain applications. Without wishing to be bound by any theory, it is believed that known processes have insufficiently coordinated the addition of nitrogen with the sintering temperature so that the recrystallization process is coordinated with the supply of nitrogen to provide the required degree of nitrogen incorporation for certain igniter applications.
Thus, a need has arisen for method of making a hot surface igniter which addresses the foregoing.